April Fool's Day or All Fool's Day occurs annually on April 1. The day is generally observed by playing a practical joke on a "victim" who soon becomes known as an April Fool. This custom is thought to have started in France during the 16th century but the British are credited with bringing it to the United States.
The commonly accepted origin of April Fool's Day involves changes in the calendar. At one time, the New Year celebration began on March 25 and ended on April 1. However, in 1582, King Charles IX adopted the Gregorian calendar and accepted the beginning of the new year as January 1. Those who refused to acknowledge the new date or simply forgot received foolish gifts and invitations to nonexistent parties. The butt of such a prank was known as a "poisson d'avril" or "April fish."